1. Field of the Invention
The present invention relates to the field of cold cathode fluorescent lamp dimming controllers.
2. Prior Art
Cold cathode fluorescent lamps are now commonly used for backlighting in notebook and laptop computers, car navigation displays, LCD monitors, point of sale terminals and portable medical equipment. In these various applications, it is generally desired to provide a dimming capability for the fluorescent lamp to provide suitable contrast under any of the varied lighting conditions that may be expected to be encountered. In the case of battery powered devices such as notebook and laptop computers, dimming has the added advantage of reducing power consumption, extending the operation of the computer before a battery recharge is required.
The dimming of fluorescent lamps presents special problems not found in incandescent lamps. By way of example, incandescent lamps may be dimmed by merely reducing the voltage applied to the lamp. In the case of incandescent lamps however, for DC operation, fluorescent lamps have an operating voltage that is not a strong function of the lamp current. Consequently, reducing lamp voltage as a primary control will simply allow the lamp to go out. Dimming can be achieved, however, by powering the fluorescent lamp with alternating power through an appropriate load or ballast device, such as an inductor, and controlling the current through the inductor and the fluorescent lamp for dimming purposes. This works well over a reasonable dimming range, and accordingly, is one of the fluorescent lamp control techniques commonly used. However, such analog dimming has a practical limit of 5:1 to 10:1 due to the thermometer effect. The thermometer effect causes the high voltage end of the tube to be significantly brighter than the low voltage end of the tube at low brightness. The tubes also have a tendency to go out and to flicker at low lamp currents.
Another technique for controlling the brightness of a fluorescent lamp is to allow the lamp to go out and periodically re-ignite the same. In that regard, a cold cathode fluorescent lamp requires a relatively high voltage for the initial breakdown and ionization of the gas in the lamp, after which the ionization will be sustained with a lower lamp operating voltage. Allowing the lamp to go out periodically (stop fluorescing), will require repeated restarting of the lamp, requiring a higher starting voltage dependent upon how long the lamp is allowed to be off. Still, periodic restarting of the lamp is not particularly difficult, and accordingly, lamp intensity can be controlled by periodically pulsing the lamp on. This low frequency chop-mode dimming is done by duty-cycle modulating the tube current at low frequency. Widest dimming range is achievable by lowering the frequency of the duty-cycle modulation and by minimizing the shortest reproducible xe2x80x9conxe2x80x9d pulse. However the frequency cannot be lower than 100 Hz because of the human eye""s perception of flicker. Also the duration of the shortest xe2x80x9conxe2x80x9d pulse is limited by how fast the system""s regulation loop can settle to create a reproducible pulse of light on the bulb. For a fast loop, the shortest xe2x80x9conxe2x80x9d pulse is about 1 ms to 500 xcexcs. This limits the dimming range of chop-mode dimming from 10:1 to 20:1.
The present invention provides a closed-loop control of the transformer voltage in a Royer inverter for cold cathode fluorescent lamps to provide a wide dimming range capability, such as by way of example, an approximately 100:1 dimming range. This dimming range is achieved by a combination of current control and pulse control to provide a much wider dimming range than can be achieved by using either form of control alone. Other features and capabilities of the invention are also described.